Chronic obstructive pulmonary disease (COPD) is a progressive disease characterized by lung injury and inflammation. Smoking cessation can slow disease progress; however, not all people benefit from quitting and many continue to have airway inflammation and significant respiratory morbidity despite abstinence from smoking. At present, the current therapeutic strategy focuses on control of illness through medications and avoidance of factors that exacerbate the disease. Although outdoor air pollution has known adverse effects on COPD morbidity and mortality, the indoor environment is of particular concern as most people spend the majority of their time indoors and at the individual level, indoor air, unlike the outdoor air, can be modified; thus strategies can emphasize reduction of pollutant exposure in the patient's home. Importantly, recent evidence from our research group shows that increased indoor air pollutant concentrations (particulate matter (PM) and nitrogen dioxide (NO2)) in homes of formers smokers with COPD are associated with respiratory morbidity, including increased respiratory symptoms, worse quality of life and increased risk of exacerbations and health care utilization. Our group has shown that home interventions that use portable air cleaners with HEPA filters successfully reduce PM concentrations and an additional carbon filter in the air cleaner may reduce NO2 concentrations. We have also shown that air cleaner intervention strategies improve respiratory symptoms in other chronic respiratory diseases, including in children with asthma, however it remains unclear whether it will work for adults with COPD. Similar studies of the efficacy of placement of portable air cleaners in homes of individuals with COPD to reduce in indoor pollutants and improve COPD health have not been conducted. In order to bridge this gap from epidemiologic evidence to research that can inform clinical practice, we propose a home intervention study to test whether targeted reductions of indoor pollutant concentrations in homes of former smokers with COPD will improve respiratory health. These aims will be accomplished using a randomized controlled trial (n=120), with 60 former smokers with COPD in each group receiving either placement of HEPA and carbon filter air cleaners or sham air cleaners in homes. We aim to determine whether an air cleaner intervention will improve respiratory symptoms, quality of life, lung function and reduce risk of exacerbations (Specific Aim #1); and whether it will be associated with intermediate outcome measures known to be linked with long term outcomes in COPD, including airway (induced sputum) and systemic (serum and urine) markers of inflammation and oxidative stress (Specific Aim #2). Our study aims build upon a well- established infrastructure of investigators with complementary areas of expertise. We anticipate that results from this study will inform clinical practice guidelines and health care policies aimed at reducing COPD morbidity. With COPD being the third leading cause of death in the US, developing strategies to reduce morbidity is an important public health priority.